1. Field of the Invention
This invention relates generally to a new form of solid supra-molecular alkylalumoxane that can be used as a catalyst or catalyst component. A new form of alkylalumoxane may be prepared using the carboxylate-substituted aluminum-oxygen nano-particles as a template. The invention particularly relates to a new heterogeneous solid catalyst which may be used as a catalyst component for the polymerization of organic monomers or in combination with a transition metal for the polymerization of olefins.
2. Description of the Related Art
The term "alumoxane" is used to describe a molecular species containing at least one oxo group (O.sup.2-) bridging (at least) two aluminum atoms, i.e., a compound containing an Al--O--Al sub-unit. The simplest alumoxane compounds are those containing two aluminum atoms bridged by a single oxygen with n additional ligands (X) bonded to aluminum (FIG. 1), Gurian et al. (1991), Wynne (1985), Kushi and Frenando (1969). Within the overall definition, the term "alumoxane" is commonly used, (especially with regard to metallocene catalysis), to denote compounds in which the pendant groups on aluminum are organic radical substituents, i.e., alkylalumoxanes. Although alkylalumoxanes are also simply referred to as alumoxanes, this class of compounds actually includes such materials as sol-gels and antiperspirants with no co-catalytic activity. Alumoxanes are also known with a variety of organic and inorganic substituents. For example, carboxylates (RCO.sub.2), alkoxides (RO) and chlorides (Landry et al. 1993).
Alkylalumoxanes are, therefore, oligomeric aluminum compounds which can be represented by the general formulae [(R)Al(O)].sub.n and R[(R)Al(O)].sub.n AlR.sub.2. In these formulae, R is an alkyl group, such as methyl (CH.sub.3), ethyl (C.sub.2 H.sub.5), propyl (C.sub.3 H.sub.7), butyl (C.sub.4 H.sub.9) or pentyl (C.sub.5 H.sub.11) and n is an integer. Such compounds can be derived from the hydrolysis of alkylaluminum compounds, AlR.sub.3. Recent reports indicate that alkylalumoxanes of the formula [(R)Al(O)].sub.n are generally cage compounds (FIG. 2), Mason et al. (1993) Harlan et al. (1994). It should be noted that while "alkylalumoxane" is generally accepted, alternative terms are found in the literature, such as: alkylaluminoxane, poly(alkylalumoxane), poly(alkylaluminum oxide), and poly(hydrocarbylaluminum oxide). As used herein, the term alkylalumoxane is intended to include all of the foregoing.
Alkylalumoxanes have been prepared in a variety of ways. They can be synthesized by contacting water with a solution of trialkylaluminum, AlR.sub.3, in a suitable organic solvent such as an aromatic or an aliphatic hydrocarbon. Alternatively, a trialkylaluminum can be reacted with a hydrated salt such as hydrated aluminum sulfate. In both cases, the reaction is evidenced by the evolution of the appropriate hydrocarbon, i.e., methane (CH.sub.4) during the hydrolysis of trimethylaluminum (AlMe.sub.3). While these two routes are by far the most common, several "non hydrolysis" routes have been developed.
Conceptually, the simplest route to alkylalumoxanes involves the reaction of water with a trialkylaluminum compound. Simply reacting water or ice (Winter et al. 1995) with an aromatic or aliphatic hydrocarbon solution of a trialkylaluminum will yield an alkylalumoxane. However, it is important to control the temperature of this highly exothermic reaction, both as a safety precaution and in order to maximize the yield and ensure the solubility of the products (Sakharovskaya et al. 1969). Several researchers have employed hydrated salts, such as Al.sub.2 (SO.sub.4).sub.3.14(H.sub.2 O) or CuSO.sub.4.5(H.sub.2 O), as "indirect hydrolysis" sources (Razuvaev et al. 1975), since the water of crystallization in a hydrated salt reacts at a vastly decreased rate as compared to dissolved "free" water.
There is also a wide range of non-hydrolysis reactions that allow for the formation of alkylalumoxanes. Ziegler in 1956 first reported the formation of an alumoxane from the reaction of triethylaluminum with CO.sub.2. Similar product is formed from the reaction of aluminum alkyls with carboxylates and amides (Harney et al. 1974). Alkylalumoxanes may also be prepared by the reaction of main group oxides (Boleslawski et al. 1975), while alkali metal aluminates formed from the reaction of trialkylaluminum with alkali metal hydroxides react with aluminum chlorides to yield alkylalumoxanes (Ueyama et al. 1973)
Alkylalumoxanes are active catalysts in the polymerization of epoxides (Colclough, 1959, Vandenberg, 1960), aldehydes (Saegusa, 1962), olefins (Longiave, 1963), and lactones (Pajerski and Lenz, 1993). Although it has been known since the 1950's that compounds of aluminum react with water to give compounds containing aluminum-oxygen bonds, commonly termed alumoxanes, it was not until the work of Manyik et al. (1966) that their application to olefin catalysis was fully appreciated. These workers showed that alkyl-substituted alumoxanes (alkylalumoxanes) were highly active co-catalysts for olefin polymerization in combination with compounds of the Group 4, 5, and 6 transition metal elements, including metallocenes containing the cyclopentadienyl ligand or substituted cyclopentadienyl ligands. Subsequent work by Reichert and Meyer (1973), Long and Breslow (1975) and Andreson et al. (1976) showed that the addition of water to the soluble metallocene/alkylaluminum catalyst systems resulted in a large increase in catalyst activity. The in-situ formation of alumoxanes in all of these systems was recognized, however, by the high catalytic activity of a metallocene and preformed methylalumoxane system as shown by the work of Andreson et al. (1976) and Kaminsky and Sinn (1980). Kaminsky et al. (1983) also demonstrated that zirconium metallocenes were more active than the titanium metallocenes.
Alkylalumoxane catalysts and co-catalyst systems suffer from a number of disadvantages. Alkylalumoxanes (especially those with methyl, ethyl and butyl substituents) are generally air sensitive, since they contains significant amounts of trialkylaluminum. For example, methylalumoxane ordinarily contains 5-15% of trimethylaluminum. A disadvantage in the homogeneous catalysts and catalyst systems involving transition metals is that the ratio of alkylalumoxane to transition metal compound, for example a metallocene, is about 1,000 to 1 or greater. Such large amounts of alkylalumoxane have several drawbacks. First, alkylalumoxanes such as methylalumoxane are costly to produce, making the economics of catalyst synthesis with respect to the end-product important. Second, the polymer made using such catalyst systems must be treated to remove the aluminum or inhibit its detrimental effects with stabilizers, dyes, and additives. A further disadvantage of homogeneous alkylalumoxane catalyst systems is that multiple delivery systems must be employed to introduce each of the components of the catalyst system into the reaction vessel because of the instability of liquid mixtures of the catalyst components.
Efforts to overcome these issues have included supporting or reacting methylalumoxane with traditional catalyst supports (Welborn, 1989). A typical support used is silica, either dehydrated or hydrated, or some other oxide. The alkylalumoxane is physically adsorbed on the surface of the support and then reacted with the metallocene. The alkylalumoxane may also be pre-reacted with the metallocene, and the product of this reaction further reacted with the surface of the silica. In each case, the alkylalumoxane:metal ratio is decreased with respect to the analogous homogenous catalyst system. However, in each case the supported or solid catalyst comprises particles in the range of 10-100 .mu.m, which limits the number of possible catalytic sites per unit mass (or volume) of the catalyst. It is highly desirable to produce solid or supported catalysts with an increased activity per unit mass or volume. In order for this to be possible, the solid catalyst must be as small as possible.